In the recent years, an optical transmission system using an optical amplifier, which directly amplifies a light signal without converting it it into an electric signal has been increasingly employed. For increasing the amount of light signals to be transmitted through a single optical amplifier, a wavelength multiplex type and a two-way transmission type of optical amplifiers have been proposed. Such an optical amplifier directly amplifies a light signal in analog fashion using an excitation light without converting it the light signal into an electric signal. When such an optical amplifier is used in a linear repeater, a supervisory light is provided in addition to a main signal to supervise the optical amplifier. The supervisory light is converted into an electric signal.
A conventional optical transmission system usually includes an optical transmitter, an optical switching device, etc. In such an optical transmission system each device is provided with a loop-back circuit to control the system. When a serious problem, such as a signal loss or out-of-frame occurs, the loop-back circuit detects and determines a location of the trouble. The location of the trouble would be in the optical transmitter, an optical receiver, an optical fiber, and the like.
In the conventional optical transmission system, when a bit-error occurs, an alarm is created and the current transmission line having the error is changed to an auxiliary line. Then, the element (package) that caused the trouble is detected and is changed.
According to the above-described conventional optical transmission system, even if trouble occurs at only one location on the current transmission line, an auxiliary line is used instead.
To decrease the opportunities of changing inferior devices, each device, such as an optical transmitter, an optical receiver and an optical repeater, needs to have enough margin for itself. If it does, it is difficult to have a wide range of system margin for the whole system.
In an optical transmission system using an optical amplifier, when a high power light is supplied into an optical transmission line, enough SN margin can be obtained. However, in response to the high power input light, an undesirable non-linear effect is generated in the optical transmission line. Such a non-linear effect influences deterioration of transmission quality. In this situation, when an output power of the optical amplifier is changed, it gets more difficult to have enough range of system margin.
If the transmission system does not have enough system margin, a plurality of alternative transmission lines has to be prepared. Even if the current transmission line has a small problem, the transmission line has to be changed to another one, because the system margin of each transmission line is small.